1. Field of the Invention
The present invention relates to a novel catalyst useful for the oligomerization of C.sub.4 olefins which catalyst comprises an alumina, gallia or india support impregnated with a monolayer of SiO.sub.2 and further containing a TiO.sub.2 phase dispersed on said SiO.sub.2 monolayer.
2. Description of Related Art
The conversion of C.sub.3 or C.sub.4 olefins into dimers and hydrocarbons of higher molecular weight using catalysts comprising nickel supported on silica or silica/alumina supports is known in the art. Dimers such as hexenes and octenes so produced are particularly useful for conversion by the well known oxo alcohol process into the corresponding heptyl and nonyl alcohols which may be used in the production of plasticizers, lubricating oil additives, detergents, defoamers and similar products.
Octene dimers are particularly useful in the manufacture of plasticizer alcohols. These dimers generally comprise a mixture of octene isomers having a varying degree of side chain methyl substitution per molecule present in the mixture.
N-octenes, for example, contain no side chain methyl groups; methyl-heptenes contain 1 side chain methyl group; dimethylhexenes contain 2 side chain methyl groups; and trimethylpentenes contain 3 side chain methyl groups. A mixture of isomeric octenes having a low content of C.sub.8 saturates is especially suitable for conversion into nonyl alcohols and gives rise to higher oxonation yields and better quality plasticizer alcohols.
It is known to dimerize olefins by contact with a nickel oxide catalyst at elevated temperature. For example, U.S. Pat. No. 3,649,710 describes a process in which butene and propylene are first pre-treated and then co-dimerized by passing over a nickel oxide catalyst. The reference teaches that the pre-treatment of the olefin substantially improves the life of the catalyst. U.S. Pat. No. 3,658,935 discloses a process for preparing propylene and n-butene dimers by contacting the feed under reaction conditions with a catalyst comprising a silica alumina gel containing 10 to 45% alumina and 0.1 to 35% nickel. In addition, U.S. Pat. No. 3,557,242 discloses a process for copolymerizing isobutene and a lower olefin using a catalyst comprising jointly coprecipitated nickel, silicon and aluminum oxide species wherein the catalyst contains from 2 to 12 wt. % nickel, 0.4 to 5 wt. % aluminum, 0.05 to 0.8 wt. % alkali metal and up to 5 wt.% coprecipitated magnesium oxide.
British Specification No. 1069296 discloses the production of dimers from olefins such as butene by contact with a catalyst containing aluminum and nickel ions on a silica support at temperatures up to 400.degree. C. British Specification No. 1215943 discloses the dimerization of olefins, including butenes, by contact with the same kind of catalyst which is activated in a slightly different manner. The catalyst used in these specifications differs from those used in the United States Patents referred to above primarily by including only a small proportion of nickel in the catalyst composition, generally less than 10% by weight based on the weight of silica gel.
In addition, U.S. Pat. No. 2,581,228 discloses a catalyst useful for polymerizing olefins which comprises a silica gel impregnated with a nickel salt and an aluminum salt such that the activated catalyst contains from about 0.1 to 35 wt. % nickel in the form of nickel oxide and from about 1 to 10 wt. % alumina based on the weight of silica gel.
Such known nickel-containing catalysts and methods for dimerizing lower olefins such as butene into higher olefins such as octene suffer from one or more disadvantages. Nickel-containing catalysts are generally more susceptible to poisoning by components present in hydrocarbon feeds such as sulfur-containing compounds. This poisoning can quickly deactivate the catalyst after only short polymerization runs. In addition, the use of such nickel-containing catalysts leads to the production of high levels of saturate compounds, i.e., octanes and isooctanes which renders the processes using these catalysts less economical. Further, the percent conversion of olefin to dimer and higher products using such catalysts may be low and the conversion per pass over the catalyst used to form the more valuable dimer products such as octenes is often too low, generally less than 50%. While the yield of octenes may be increased using dimerization process conditions including relatively low space velocity (longer catalyst contact time) and higher temperatures, it is found that the octenes produced are more highly branched, having an average content of side chain methyl substituent groups in excess of about 2. Nickel-containing catalysts are also relatively expensive and difficult to handle.